Conventionally, in places, for example, transportation systems such as an aircraft, a ship, or a train in which an amount of electric power to be used is limited, a distribution system of power supply in which the amount of electric power to be supplied to a plurality of power outlets is limited is employed.
In such a distribution system of power supply, for example, Japanese Translation of PCT Publication No. 2000-502556 discloses the following technique: since electric power cannot be supplied beyond the maximum allowable power of the power supply, when the total amount of electric power used at a plurality of power outlets exceeds the maximum supply amount of the power supply, other power outlets are disabled, thus suppressing further increase of supply of electric power.
Hereinafter, technical details of a distribution system of power supply disclosed in Japanese Translation of PCT Publication No. 2000-502556 are described with reference to FIG. 5. FIG. 5 is a block diagram showing a load distribution and management system. As shown in FIG. 5, power distribution circuit 12 receives electric power from power supply 16. Power distribution circuit 12 distributes the received electric power to power units 14a to 14n each having a power outlet. Electronic devices 32a to 32n and electronic devices 33a to 33n are connected to the power outlets of power units 14a to 14n, respectively.
Power distribution circuit 12 includes power sense circuits 34a to 34n. Each of power sense circuits 34a to 34n measures an amount of electric power drawn by each of power units 14a to 14n. Then, power sense circuits 34a to 34n communicate information about the amount of the drawn electric power to power comparator 38.
Each of power sense circuits 34a to 34n also determines whether or not each of power units 14a to 14n exceeds the upper limit value of respective maximum electric power. For example, if it is determined that the amount of electric power being drawn by power unit 14a exceeds the upper limit value of the maximum electric power of power unit 14a, power sense circuit 34a generates a signal.
Power comparator 38 receives the amount of electric power being drawn by each of power units 14a to 14n. Power comparator 38 calculates a total amount of electric power being drawn by summing inputs 46a to 46n. Then, power comparator 38 compares the total amount with threshold input from maximum load circuit 40.
If the total amount of electric power is larger than the threshold value determined by maximum load circuit 40, with respect to each of power units 14a to 14n which are not being used, power units 14a to 14n are stopped via power available circuits (in FIG. 5, simply referred to as “circuit”) 36a to 36n, so that supply of electric power cannot be received from the power outlets of the stopped power units.
That is to say, in the above-mentioned configuration, power distribution circuit 12 inhibits the use of other unconnected power units. Furthermore, power distribution circuit 12 supplies electric power preferentially to a firstly connected power unit, and may stop supplying of electric power to a later connected power unit even if it is being used.
Therefore, in a conventional power supply distribution system, when the use of the power outlet is inhibited, even when power consumption is small, for example, when a small device such as a portable telephone is charged, electric power is not supplied. Therefore, convenience for a user is lost.